Evaluation of Fully Biodegradable PLA/PHB Blend Filled with Microcrystalline Celluloses

In this work, biodegradable biocomposites were developed using PLA/PHB blend as matrix and two types of microcrystalline cellulose as filler at three different contents. The biocomposites were evaluated regarding their thermal and morphological characteristics and molecular dynamic behavior. It was seen that cellulose addition did not promote significant changes in the Tm, Tc and Tcc in the matrix. On the other hand, XRD and TGA revealed that the addition of the highest content (7 wt%) of cellulose fillers resulted in a more significant decrease in crystallinity and thermal stability of the PLA/PHB matrix, suggesting a formation of filler aggregates. This indication was confirmed by TD-NMR, whose results pointed to a greater heterogeneity molecular in the samples containing higher cellulose contents. Therefore, this technique proved to be a relevant and complementary tool for the characterization of composites materials, contributing to determinate the most appropriate filler content introduced in a polymer matrix.


Introduction
Biodegradable polymers from renewable resources are the most promising alternative to replace conventional petrochemical polymers and minimize the environmental impacts caused by them, such as the accumulation of plastic waste in landfills and their penetration and contamination in the form of microplastics into the whole ecosystem 1 .For this reason, research related to the development of fully biodegradable "green" materials has increased massively in recent years 2 .
This class of polymers has many relevant advantages over those conventionally obtained from petroleum, such as biodegradability, biocompatibility, low toxicity, sustainability, among others 3,4 .However, the wide application of these polymers is still a challenge due to inherent limitations on their performance compared to conventional polymers, which in general exhibit superior mechanical properties and thermal stability 5 .These limitations can restrict their use to shortterm and single-use applications, such as food packaging.
Among the biodegradable polymers, the aliphatic polyesters are the most attractive due to their good mechanical properties, processability and the ability to undergo both hydrolytic degradation and biodegradation by soil microorganisms in compost 6 .Poly(lactic acid) (PLA) and poly(hydroxyalkanoates) (PHAs) such as Poly(3-hydroxybutyrate) (PHB) are examples of these materials and typify biodegradable polymers derived from renewable resources.Furthermore, both of them are commercialized at large scale, therefore they are suitable candidates for the development of materials that demand high production volume, such as for packing applications.
Polylactic acid or polylactide (PLA) is an aliphatic thermoplastic polyester produced from renewable resources.It is a biodegradable polymer [7][8][9][10] that has been employed for various applications, i.e. biomedical, packaging, textile fibers and technical items 7 .PLA is industrially obtained through the polymerization of lactic acid (LA) or by the ring-opening polymerization (ROP) of lactide (the cyclic dimer of lactic acid, as an intermediate) 7,11,12 .Among its attractive properties, it can be pointed to high transparency, high rate of disintegration in compost, ease of processing and ready availability 13 .In addition, PLA exhibits satisfactory mechanical properties (particularly, high tensile strength and Young's modulus and acceptable flexural strength), which are even higher than of many commonly used polymers, such as polystyrene (PS), polypropylene (PP), polyethylene (PE) and others 7,14 .The tensile strength and elastic modulus of PLA are comparable to those of PET, which lead this polymer to a key position in the market of biopolymers, being one of the most promising candidates for further developments in this area.Unfortunately, PLA suffers from some shortcomings, such as being sensitive to moisture, having low-impact strength and being notably brittle, with less than 10% elongation at break and low toughness, which limits its use in applications that demands plastic deformation under high stress 15 .These drawbacks can be surpassed through blending with other polymers and/or through developing bionanocomposites, which can lead to the tuning of their final properties 7 .
Poly(3-hydroxybutyrate) (PHB) is an aliphatic polyester with linear polymer chain and it is the predominant polyhydroxyalkanoate (PHA) synthesized by controlled bacterial fermentation 31 .Microbial synthesis of PHB is the preferred method for industrial production because it ensures the proper stereochemistry for biodegradation.Microorganisms synthesize and store PHB when nutrient limited conditions are imposed, while degrading it and metabolizing as the limitation is removed 32,33 .Current production employs Alcaligenes eutrophus because it grows efficiently on glucose as a carbon source, accumulates PHB up to 80% of its dry weight, and is able to synthesize polyhydroxybutyrate-co-valerate (PHBV) when propionic acid is added to the feedstock 33 .
PHB presents a high degree of crystallinity, which is an important feature to improve PLA's properties, in addition to having superior physical properties over polypropylene for food packaging applications and being completely nontoxic.Furthermore, PHB is optically active, presents a good barrier to permeability of water and gases and exhibits acceptable stability to ultraviolet radiation 34 .On the other hand, it is a brittle polymer, exhibiting inferior low-impact strength, and its poor processability and thermal instability when processed are the foremost drawbacks that limit its industrial usage 30 .The literature proposes the blending, the development of copolymers or the insertion of additives as strategies to improve the mechanical and thermal properties of PHB 34 .Another alternative is the development of PHB nanocomposites due to the potential for improving their properties 35 .
Several studies describe PLA/PHB blend as a valuable approach to produce "green" materials, since its properties can be easily modulated through changes in composition.However, some properties still need to be improved to broaden the range of applications.In this context, incorporation of reinforcement materials can promote improvements in this blend properties.Thus, the development of bio-based materials with natural reinforcement fillers, such as cellulose, starch, and chitin, appeals as a promising strategy to provide the enhancement of their properties 36 , without interfering with the total biodegradability of the produced material.
Among these fillers, cellulose particularly has been largely employed to produce biocomposites, considering it is the most abundant biopolymer in nature and is available in a wide variety of resources, such as plants and microorganisms.The isolated cellulose should be submitted to a partial acid hydrolysis process to produce microcrystalline cellulose.During the process of acid hydrolysis, the non-crystalline region is preferentially hydrolyzed to produce a cellulosic material with high crystallinity 37 and better mechanical properties.
The purpose of this study was to develop fully biodegradable biocomposites based on the blend of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB) filled with microcrystalline cellulose untreated and treated by sonication at 3, 5 and 7 wt% and to characterize the obtained materials by conventional techniques, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) and also by a more recent and unconventional technique named time domain-nuclear magnetic resonance (TD-NMR).This characterization provides relevant and complementary information about the molecular dynamic behavior of the materials through the nuclear relaxation measurements such as by spin-lattice relaxation time (T 1 H) determination and regarding the homogeneity at molecular level by means of domain distribution curve profile, allowing a more detailed evaluation of the composites systems 38 .Furthermore TD-NMR allows to conduct measurements fast and without any special sample preparation.

Materials
The materials used in this study were supplied as follows: NatureWorks™ 2002D PLA in pellet from Nature Works; Biocycle ® , PHB in powder form from PHB Industrial S.A., Chloroform (CHCl 3 ) from Merck Chemical Company, Microcrystalline cellulose (MCC) ph102 in powder form from Viafarma; and sMCC obtained from MCC by sonication treatment.

Preparation of the microcrystalline cellulose
The sMCC (sonicated MCC) was produced from MCC aqueous suspensions at a concentration of 1 wt% by subjecting them to high intensity ultrasonication treatment.The suspensions were exposed to ultrasonication for 60 min at 25 ºC in order to modify the cellulosic material's crystalline structure by size reduction and shape modification of its crystallites.Afterwards, the ultrasound irradiated suspensions were freeze-dried for 48 h to obtain powdered sMCC.The procedure was carried out in Eco-sonics equipment from Ultronique Company, Disruptor model, at a frequency of 20 kHz and potency of 500 W.

Preparation of the blended biocomposites systems
The blended polymeric films were prepared by the solution casting method using CHCl 3 as solvent.Two different series of materials were obtained by this method, both based on a blended polymeric matrix made of PLA and PHB.These series differ in the reinforcement filler type added to them.The first series was reinforced with MCC, used as received, and the second one with sMCC (sonicated MCC).
For the formulation of each film, PLA and PHB at a ratio of 3:1 (wt%) were solubilized simultaneously in chloroform (CHCl 3 ) under vigorous magnetic stirring for 24 hours at room temperature, resulting in a 10% w/v solution.Formulations for neat PLA and PHB were also prepared by the same method for the production of individual films of each polymer.
Afterwards, MCC or sMCC were systematically added to chloroform in three different proportions, resulting in three dispersions of each cellulosic filler, appropriate to produce films with 3, 5, and 7 wt% (filler/polymers blend).In this procedure, the cellulosic fillers were dispersed in chloroform by magnetic stirring at room temperature for 30 min, followed by a sonication bath for another 30 min.Subsequently, each polymer blend solution was added to each cellulosic filler suspension and kept under constant magnetic stirring for an additional hour.The resulting mixtures were cast into glass Petri dishes and placed in the fume hood for at least three days to evaporate all residual solvent.A polymer blend film without cellulose fillers was also prepared by this method.The produced films were coded as described in Table 1.

Characterization of the microcrystalline cellulose fillers
The morphology of MCC and sMCC samples was investigated by Scanning Electron Microscopy (SEM).For the sample preparation, each cellulosic material in powder form was individually placed on a double-sided carbon tape and gold coated for 4 min in sputtering.The equipment used for this analysis was a Hitachi TM3030 Plus Scanning Electron Microscope at an accelerating voltage of 1-15 kV.

Characterization of the blended biocomposites systems
The characteristics and properties of the blended biocomposites systems were evaluated by X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Time Domain-Nuclear Magnetic Resonance (TD-NMR).The analyses were performed as described below.
XRD was performed using Rigaku Ultima IV diffractometer with CuKα radiation generator (λ=0.154nm, 40Kv, 120 mA) at room temperature, in the range of 2θ from 2° to 40° at a rate of 1°/min, and step of 0.05°.This technique was used to evaluate the crystalline profile of the materials.Their crystallinity degrees were determined using Origin® software, according to the equation X C (%) = I C / (I C + I A ) x 100, where Xc is the crystallinity degree; Ic is the sum of the areas under the crystalline peaks and I A is the area of the amorphous halo.The peaks were deconvoluted using Gaussian peak function.
DSC analyses were carried out using a TA Instruments Q1000 calorimeter (with a temperature accuracy of ±2 ºC) under a nitrogen flow rate of 50 mL/min.The samples were subjected to a first heating ramp from -30 to 200 ºC, followed by a cooling ramp from 200 to -30 ºC.After this heating/cooling cycle, the samples were subjected to a second heating ramp from -30 to 200 ºC.The heating and cooling ramps were all performed at a scanning rate of 10 ºC/min.The crystallization temperature (Tc), cold crystallization temperature (Tcc) and melting temperature (Tm) were determined from second cooling and second heating scans.The first heating ramp was used only to erase the polymer thermal history.
TGA measurements were performed using a TA Instruments Q500 calorimeter (with a temperature accuracy of ±2 ºC).The samples were placed in a platinum holder under continuous nitrogen flow and heated at the rate of 10 ºC/min from 20 to 700 ºC.This analytical technique was used to investigate the thermal stability of the produced materials.From TGA two parameters were measured to study the thermal stability of the prepared materials: the initial degradation temperature (Tonset) and temperature of maximum degradation rate (Tmax).The Tonset values were obtained from TG curves and denote the lowest temperature at which mass variation of the material occurs.In turn, the Tmax values were obtained from the peak of each DTG curve, which refers to the temperature where the degradation speed occurs more sharply.
TD-NMR analyses were performed using a low-field NMR spectrometer Maran Ultra operating at 23 MHz, employing an inversion-recovery pulse sequence (recycle delay -180º-τ -90º -acquisition time).The analysis was carried out at 27 °C; with τ values varying from 0.01 to 10,000 ms and recycle interval of 3 s, utilizing 40 points with 4 accumulations.The equipment was operated to determine the spin-lattice relaxation times of the hydrogen nucleus (T 1 H) and the distribution domain curves.The spin-lattice relaxation times were obtained with the aid of the WinFit program and the distribution domain curves were fitted with the WinDXP software.

Characterization of the microcrystalline cellulose fillers
SEM analyses were performed to investigate MCC morphology and describe possible changes after the ultrasonication treatment.Images obtained by SEM for the cellulosic fillers (Figure 1) revealed that the ultrasonication treatment of MCC promoted modifications in the size and shape of the particles.As observed in the SEM images, the MCC particles predominantly presented a more regular morphology with long or rounded structures, having an average particle size around 100 µm (Figures 1A and 1B).On the other hand, sMCC presented reduced size particles, around 25 µm, with shorter structure and irregular morphology compared to the MCC particles (Figures 1C and 1D).Furthermore, the SEM images for sMCC sample exhibited aggregates formed by its thinner particles, as highlighted in Figures 1C and 1D.
For the neat PLA film, it was observed an amorphous halo (Figure 2), indicating low degree of crystallinity of this film.This result is expected for PLA films obtained by solution casting method and it is consistent to other reported in the literature 41,42 .
The result obtained from XRD analysis for the neat PHB film revealed a crystalline profile corresponding to orthorhombic crystal planes.For this film, two strong scattering intensity peaks were detected at around 2θ = 13° and 17°, which are assigned to the (020) and (110) planes of the orthorhombic unit cell, respectively.Both correspond to the characteristic peaks of PHB crystallinity.Other weaker reflections located at around 22.5°, 26°, 27° and 31° correspond to (111), ( 121), ( 040) and (002) planes, respectively.Furthermore, the result of this analysis also showed that the neat PHB film presented a small amount of orthorhombic β-form crystal with zig-zag conformation, as revealed by the reflection of the (021) plane located at 2θ = 20°4 3 .This result showed that unlike PLA, the solution casting method did not prevent the development of crystallinity in the PHB film.
The XRD pattern of PLA/PHB blend (Figure 2) showed peaks at around 13º, 22.5º, 26º and 27º characteristic to the PHB, but weaker than that found for neat PHB film.Presumably, this decrease in the intensity of the peaks is due to the minor quantity of PHB (25 wt.%) in this blend formulation.For the same reason, it was not possible to identify the peak found in the pattern of the neat PHB film at around 31°. Furthermore, the XRD pattern of the unfilled PLA/PHB showed peaks at 16.9° and 19.3º, both attributed to the PLA phase 44,45 .The very strong reflection at 2θ = 16.9°corresponds to (110) and/or (200) planes, while the less intense peak at 19.3° is assigned to the reflection of the (203) plane 46,47 .These diffraction peaks indicated that the addition of semi-crystalline PHB induces the PLA's crystallinity, suggesting that PHB acts as a nucleating agent in PLA, which is in accordance with other reports [48][49][50] .The nucleating effect observed can be attributed to the highly ordered stereochemical structure of PHB crystallizes as small spherulites that are well dispersed in the amorphous PLA matrix and act as nucleating agents for this polymer 50 , increasing its crystallinity 51,52 .
Regarding to the PLA/PHB biocomposites it was found a similar diffractogram profile compared to PLA/PHB blend unfilled (Fig. 3), but it was also possible to identify a more prominent peak at 2θ = 22,5º, suggesting the contribution of the (200) plane of cellulose fillers.Similarly to the observed in the XRD pattern of PHB film, all the PLA/PHB biocomposites samples presented a small amount of orthorhombic β-form crystal with zig-zag conformation, as revealed by reflection at 2θ = 20º.Compared to the PLA/PHB unfilled film, PLA/ PHB/MMC and PLA/PHB/sMCC systems containing 7 wt.% of cellulose filler presented a slight reduction in the peak intensity at 16.9º, suggesting changes in the crystalline profile of the matrix (Figure 3A and Figure 3B).
Regarding degree of crystallinity, it was found values of 49% for PLA/PHB unfilled, while PLA/PHB biocomposites containing 3%, 5% and 7% of MCC filler presented values of 43%, 42% and 38%, respectively and PLA/PHB biocomposites containing 3%, 5% and 7% of sMCC exhibited values of 46%, 45%, and 41%, respectively.This result showed that the progressive addition of both MCC and sMMC fillers promoted a gradual decrease in the crystallinity of the matrix.However, comparing the materials with the same cellulosic filler content, the crystallinity values of the films containing sMCC were higher than those found for the films with MCC.This result suggests that MCC and sMCC have a slightly different influence on the crystalline profile of the PLA/PHB matrix.This effect can be related to the different morphologies of these cellulosic fillers.The sMCC particles presented shorter structures than the MCC ones, which indicates that sMCC has a higher specific surface area.This characteristic of sMCC probably contributes to increasing the contact area between the surface of sMCC particles and the polymers.Thus, the most pronounced effect on the crystalline profile of PLA/PHB matrix was promoted by sMCC incorporation, which is possibly related to the more effective interaction between both phases in the biocomposites.

Differential scanning calorimetry (DSC)
The data obtained by DSC for neat PLA, neat PHB, PLA/ PHB blend; PLA/PHB/MCC biocomposites and PLA/PHB/ sMCC biocomposites are denoted in Table 2.The parameters include: melting temperature (Tm); melting enthalpy (ΔHm), crystallization temperature (Tc) and cold crystallization temperature (Tcc).For this discussion, Tm1 refers to the main melting temperature peak found for both PLA and PHB and the melting enthalpy (ΔHm1) corresponds to the Tm1.In addition, Tm2 and Tcc1were exclusively related to the PLA phase, while Tcc2 was related to the PHB phase.
Regarding unfilled polymer materials, the results showed that neat PLA did not crystallize during cooling, thus it did  not present crystallization temperature (Tc) as was also observed in other studies 53,54 .On the other hand, it was possible to identify a crystallization event on heating (cold crystallization, Tcc1) at 99 ºC (Table 2).Furthermore, PLA presented two melting temperatures: Tm1 with an intense endothermic peak at 174 ºC related to α crystal form and Tm2 with an almost inconspicuous peak at 160 ºC referred to β crystal form.The melting temperature of the α-form is higher because of the better quality and higher size of its crystals 54 .
Concerning the PHB it was found a notable exothermic peak during cooling at 65 ºC related to crystallization temperature (Tc), while Tm was detected at 174 ºC.Moreover, it was identified an exothermic peak during the second heating due to the cold crystallization temperature at 48 ºC (Tcc2).This parameter was also reported elsewhere 55 .
PLA/PHB blend, as well PLA, did not present Tc, showing that PHB addition did not favor the crystallization on cooling.This sample presented only cold crystallization temperatures: Tcc2 at 46 ºC related to the PHB phase; and Tcc1 at 95 ºC related to the PLA phase.Ultimately, the Tm1 was found at 174 ºC (Table 2).
For the developed biocomposites, except for PLA/PHB/MCC5 and PLA/PHB/sMCC3, it was observed that both MCC and sMCC induced crystallization on cooling (Tc).This result indicates that the microcrystalline cellulose fillers used in this study can act as nucleating agent for this crystallization mode.The addition of fillers promoting a nucleating effect for PLA has been reported in other studies 53,56 .Regarding melting temperature (Tm), the addition of MCC and sMCC at any of three proportion maintained the values found for PLA/PHB blend (Table 2).

Thermogravimetric analysis (TGA)
TGA analysis is a very useful tool to investigate the thermal stability of materials.In this study, two parameters were measured to evaluate the thermal behavior of the prepared biocomposite systems and then compare them to the PLA/PHB unfilled matrix.The first parameter obtained from TG curves was the Tonset value, which represents the lowest temperature at which mass variation of the material occurs.The second parameter was the Tmax value obtained from the DTG peak, related to the temperature of maximum degradation rate.
Comparing the thermal stability profile of neat PLA and neat PHB, it was observed that both materials presented a single-step thermal degradation (Figure 4).However, PLA had a higher thermal stability than PHB with Tonset at 314 ºC and Tmax at 345 ºC, while PHB showed Tonset at 256 ºC and Tmax at 270 ºC (Table 3).Regarding the PLA/PHB blend, it was observed that the addition of PHB caused changes on the thermal degradation profile.The TGA curves of the PLA/PHB blend revealed a two-stage degradation.The first step of mass loss was attributed to the degradation of PHB and the second stage was related to PLA, as also described elsewhere 57 .For this reason, it was named Tonset 1 and  Tmax 1 for temperatures related to PHB phase and Tonset 2 and Tmax 2 for those concerning to PLA phase.PLA/PHB blend presented Tonset 1 and Tmax 1 at 270 ºC and 280 ºC, respectively and Tonset 2 and Tmax 2 at 320 ºC at 354 ºC, respectively (Figures 4A and 4B).Considering the thermal decomposition of the PLA and PHB phases separately, it was possible to observe a slight increase in the thermal stability of both phases in the blend, so that Tonset 1 and Tonset 2, as well as Tmax 1 and Tmax 2 presented higher values in the PLA/PHB blend compared to the values found for neat PLA and PHB (Table 3 and Figure 4B).
For the biocomposites, MCC addition at 3 and 5 wt% did not cause a significant change in the thermal stability of the PLA/PHB matrix, maintaining Tonset 1, Tonset 2, Tmax 1 and Tmax 2 practically unchanged.On the other hand, the addition of 7 wt% of MCC promoted a significant decrease on the thermal performance of the PLA/PHB matrix, with a decrease of 35 °C and 27 ºC in the Tonset 1 and Tmax 1, respectively and of 19 ºC and 36 ºC on the Tonset 2 and Tmax 2, respectively (Table 3, Figure 5A and Figure 5B).
For biocomposites prepared with sMCC, the effect of filler addition was slightly different.For these systems, it was observed a decrease in the thermal stability for all formulations, but in the same way as was seen for the formulations containing MCC, for PLA/PHB/sMCC formulation containing 7 wt% of filler was observed a more significant decrease on the thermal stability.For this formulation, there was a decrease of 23 ºC and 18 ºC related to Tonset 1 and Tmax 1, respectively and of 15 ºC and 32 ºC on the Tonset 2 and Tmax 2, respectively (Table 3, Figure 6A and Figure 6B).The results obtained by TGA suggest that the addition of 7 wt% of microcrystalline celluloses, regardless of the type, resulted in a more heterogeneous systems with weaker interactions between the polymer and filler phases.

Time Domain-Nuclear Magnetic Resonance
(TD-NMR) The proton spin-lattice relaxation time (T 1 H) and the domain curves of the samples were obtained by an unconventional TD-NMR technique, using a low-field NMR equipment.The obtained parameters by TD-NMR provide important information about dynamic molecular behavior of materials such as polymers and their composites and nanocomposites 58 .The determination of the nuclear relaxation parameters can be exploited to discriminate different dynamical regimes inside the material, depending on its structure and morphology.Motional correlation times affect, in turn, macroscopic dynamical behavior, related to viscoelasticity, mechanical response, and processing characteristics 59 .T 1 H measurements inform about the mobility of the sample at the molecular level, so that higher T 1 H values mean lower molecular mobility.Domain curves provide information about the homogeneity of materials at the molecular level, according to the number of curves present in the plot and these curves' base width.For this parameter, fewer domain curves and narrower baseline of curves mean a more homogeneous material 60 .
Figure 7 shows the results obtained from TD-NMR analysis for neat PLA, neat PHB and PLA/PHB blend.Regarding T 1 H values, it was found that PLA presented a spin-lattice relaxation time at 724 ms, PHB at 643 ms and PLA/PHB blend at 693 ms.The highest T 1 H relaxation time found for PLA indicates that this sample has lower molecular mobility.Furthermore, it was seen that PHB incorporation into PLA to develop the blend increased the molecular mobility of this system, so a less brittle material is expected.In relation to the domain curve plot, it was noted that the PLA/PHB blend presented a wider base of the domain curve, indicating a greater molecular heterogeneity of this sample compared to isolated PLA and PHB, as expected, since it is an immiscible polymer blend.
For all PLA/PHB systems containing MCC or sMCC, regardless of filler ratio, it was observed a decrease in the T 1 H values compared to the unfilled PLA/PHB matrix (Figures 8A and 8B), which indicates a reduction of the inter-and intramolecular interaction within the polymer chains promoted by filler distribution in the matrix.
Analyzing the systems separately, for PLA/PHB/MCC systems it was found T 1 H values at 663 ms, 664 ms and 613 ms for systems containing 3, 5 and 7 wt% of MCC, respectively.Compared to the T 1 H value of the unfilled PLA/PHB (693 ms), it was inferred that the addition of MCC similarly increased the molecular mobility of the systems containing 3 and 5 wt.% of filler.For the system containing the highest MCC proportion (PLA/PHB/MCC7), the change was more significant, indicating the greater molecular mobility found among PLA/PHB/MCC systems.From domain curves, it was possible to observe that the addition of MCC at 3 wt% in the PLA/PHB matrix promoted a higher homogeneity at molecular level, as can be seen by a narrowest domain baseline found for PLA/PHB/MCC3 system, compared to other samples, including the unfilled PLA/PHB.
For the PLA/PHB/sMCC systems, were found T 1 H values at 653 ms, 635 ms and 611 ms for systems containing 3, 5 and 7 wt% of sonicated microcrystalline cellulose (sMCC), respectively.This result showed a slightly different behavior compared to those observed for the systems prepared with MCC.For the PLA/PHB/sMCC systems, it was seen a progressive decrease in the T 1 H values with increase of sMCC.This behavior indicates that the increasing addition of sMCC reduces progressively the molecular rigidity of the PLA/PHB matrix.In relation to the data obtained from the domain curves, it was seen that the increasing addition of sMCC disfavors the homogeneity of the system, as the domain curves become progressively wider or new domains appear.Similarly, to that observed for PLA/PHB/MCC systems, the sample that showed the greatest molecular homogeneity was the one containing 3wt.% of sMCC and the most heterogeneous system was the one containing 7 wt% of filler.
Comparing the systems prepared with MCC and sMCC at the same filler proportions, it was observed that the systems containing sMCC presented a more significant change, since the T 1 H values were lower and the domain distribution curves displayed a wider base curve and/or new minor domains were created, indicating greater heterogeneity.The results obtained by TD-NMR corroborated with those found by TGA and by XRD, indicating that the heterogeneity molecular of composites systems can induce a decrease of some properties such as thermal stability and degree of crystallinity of the materials.

Conclusions
Biodegradable biocomposites were developed using PLA/PHB blend as matrix and two types of microcrystalline cellulose at 3, 5 and 7 wt% as filler.The influence of the type and content of cellulose filler on the thermal properties and crystallinity parameters of the obtained samples were investigated.In addition, TD-NMR technique was carried out to examine the molecular dynamic behavior and homogeneity at molecular level of the developed materials and investigate a possible correlation of these parameters with thermal and morphological properties evaluated in this study.Generally, it was found by XRD and TGA that the highest content (7 wt%) of both cellulose fillers caused decrease in the crystallinity degree and in the thermal stability of the PLA/PHB matrix, respectively.TD-NMR analysis provided a more detailed investigation, showing that the PLA/PHB systems containing 7 wt% of MCC or sMCC exhibited greater heterogeneity at molecular level.From this result it was concluded that, despite not interfering in the thermal transition temperatures investigated by DSC, the molecular heterogeneity parameter was related to the decrease in thermal stability and degree of crystallinity of the samples.Thus TD-NMR, which is considered as a fast and non-destructive technique, can be used as an important tool in the characterization of composite materials, assisting in the determination of the best filler content in these materials, based on the evaluation of molecular homogeneity, thus allowing to optimize the final properties of the developed materials.

Figure 1 .
Figure 1.SEM images obtained for MCC (micrographs A and B) and sMCC (micrographs C and D).

Figure 7 .
Figure 7. Distribution domain curves for the PLA, PHB and PLA/PHB blend.

Table 1 .
Description and codes of the materials.

Table 2 .
Thermal data obtained by DSC for PLA, PHB.PLA/PHB blend and their biocomposites systems.

Table 3 .
TGA data obtained for PHB, PLA, PLA/PHB blend and their biocomposites systems.
Figure 5. TG curves (A) and DTG curves (b) obtained for PLA/PHB/MCC biocomposite systems.Materials Research